The present invention relates to crystal oscillators, and more particularly, to a crystal oscillator structure that significantly reduces the sensitivity of a crystal oscillator to vibration.
The use of crystal resonators to control an oscillator is well known. The frequency of the crystal controlled oscillator is primarily depended upon the natural resonant frequency of the crystal resonator. However, the natural resonant frequency of the crystal resonator changes when acceleration forces are applied to the crystal resonator. This problem and prior attempts at solving the unwanted frequency changes due to these acceleration forces are shown in, for example, U.S. Pat. No. 4,410,822 to Filler: U.S. Pat. No. 4,575,690 to Walls et al.: and U.S. Pat. No. 4,851,790 to Driscoll. In both U.S. Pat. No. 4,575,690 and 4,410,822, the acceleration sensitivity vectors of respective crystal resonators need to be matched. In addition, the crystal resonators need to be aligned so that their respective acceleration sensitivity vectors are positioned in very accurate orientations with respect to each other. These approaches are not easily implemented, are subject to alignment errors and are subject to alignment changes due to vibration. As a result, the prior approaches were very expensive and not practically implementable.
U.S. Pat. No. 4,851,790 discloses a structure that mechanically isolates a crystal resonator and associated sustaining stage. The crystal resonator is then mounted on vibration dampers. The improvement that this approach provides depends, in large part, upon the material used and the structure of the vibration dampers or isolators.
Another technique to reduce the vibration sensitivity of a crystal resonator is to use accelerometers to control an oscillator frequency modulation circuit to produce a frequency that changes in an equal and opposite direction to the change in frequency induced in the crystal resonator due to the applied acceleration forces. This technique results in approximately a 10:1 reduction in vibration sensitivity. This improvement, however, is limited to several hundreds hertz. This approach is also impractical due to the inability to obtain reproducible results over a wide range of vibrational frequencies; the high cost associated with obtaining accurate accelerometer alignment, and proper cancellation signal phasing as well as the inability to maintain the cancellation signals with the proper ratios due to changes in the crystal resonator and/or accelerometer vibration sensitivity with time and temperature.
Another technique used for reducing the vibration sensitivity of a crystal resonator is to use two crystals that have been carefully mounted so that their respective vibration sensitivity vectors change by the same amount in response to a given acceleration force. If the crystals are carefully mounted so that their vibration sensitivity vectors are positioned in opposite directions, the resultant resonator will have a zero acceleration sensitivity vector. This approach, however, is extremely impractical because it is almost impossible to find crystals matched as needed. The acceleration vector of each crystal would need to be carefully measured and the crystals must be carefully mounted at a very precise angle with respect to each other.